RPM question

[Jamescenterconsole]

Sorry this may end up being on the wrong forum.
Info-- 1981 Cobia center console 18ft rebuilt floor and transom added t-top, 1982 Mercury outboard 80hp cleaned carbs new gaskets replaced stator and trigger (bad wires) control box (no spark on 3 and 4)
Took the boat out the other day and it ran great. However at WOT I could only get 3200 RPMs out of it, 27MPH by GPS. It doesn't sound like it lugging or anything. Im thinking Im over propped. but I wanted another opinion before I try that, prop is seized to the shaft and I may have to cut it off. so I don't know what's currently on it prop wise.
My motor question is, Is there anything I should check motor wise it seems to rev plenty high when its in natural but I was wondering about timing and spark advance.

 

[Texasmark]

I assumed your engine has a 2.33 gearbox. Chris 1956 ought to know for sure. Ping him. With that gearbox and an assumed 20% prop slip estimation for your boat, taking your numbers of rpm and gps to the go-fast BAM prop slip calculator, you would have to be turning a 25P propeller which I seriously doubt; first being I doubt you could find one without some serious searching and it would be a SS high performance, and for another I ran a 24P high performance on my '02 90 hp with a 2.33 and it's on a very light 17' aluminum boat. For that much boat and that small an engine considering, I would assume more like a 17 or 19. According to the prop slip calculator, with either of those props your theoretical maximum speed, if the prop were turning in a solid, not slipping in water, would be 22 for the 17 and 25 for the 19.......long way from your gps number of 27 if you add any slip.

Something wrong here. Assume your gearbox is 2:1 like the larger engines of that day then running the numbers again, a 17 theor. would be 26 gps and the 19 29gps, still no slip added to reduce the actual number.

So let's assume your tach is off and you "are" running a 17P and solving for rpms with a 2:1 gearbox and estimated 20% slip gives a theoretical max for 27 actual at 20 slip is 32 mph and backing the truck on up the table, the rpms for that setup would be 3950. Even at 3950 the engine is lugging. It needs to be on up around 5000. At 5000 your engine would be singing not humming for what it should sound like.

Nothing fits here.
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Mark a line from your prop hub to your lower unit. With your shifter in F, rotate your flywheel two full 360 degree revolutions and see if the line returns to the marked position. If so you have a 2:1 gearbox. If it didn't get there and the flywheel had to turn a little over a half turn additional to get it there you have the 2.33.

Take your prop to a prop shop and have them tell you what is your pitch.
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Get the two numbers and come back and we can help you move forward!

Mark

 

[Alumarine]

If you're getting 27 mph on gps I'm guessing your tach is off a lot considering the boat/motor combo.
Is this a new to you boat?

 

[Jamescenterconsole]

Ok, Thanks for the info. Ill check the gear ratio. Also I pulled the plugs and noticed that #4 did not look like it had burned as hot as the other three. Bad coil maybe? yes this a new boat to me. guy said the motor ran, it did on two cylinders. I rebuilt everything trying to get it right now. Thanks again.

 

[Jamescenterconsole]

Ok here is the info that Texasmark and marc c asked for
gear ratio - marked the hub turned the fly wheel, it went two and a quarter turns to get the prop all the way around
tach- has 5 settings 4, 6, 8, a, and b, it is set to b which Im assuming is 12 pole?????
prop- is either 13 or 13.5 inches the number that I finally found is xx 96208, not sure on the xx's they were smashed by the nut and hard to read
motor is a 1982 Mercury, ser# 6175133, 80 hp
still trying to get the prop off, it is seized to the shaft, have a puller on it now with it soaking tried heat no luck yet
going to order 4 new coils for it tomorrow since my number four spark doesnt seem hot enough
going to borrow a service tach from a friend today or tomorrow
any new advice would be great

 

[Jamescenterconsole]

anybody have any ideas ?????

 

[Jamescenterconsole]

anybody have any ideas ?????

 

[Chris1956]

Gee, I would look up the tach and see what the correct setting for Mercury is, rather than guessing "B". I also think the "6" setting is likely to work on that motor. Once you get the prop off and are sure of the pitch, and RPM, we can suggest a replacement prop pitch. 80HP on an 18 footer is not going to be a speed demon. 27MPH may be all you get....

 

[Jamescenterconsole]

Thanks Chris, the guy at the shop where I got the Tach set it for me so I assumed it was right. And your right 80hp is not much and the speed is about what I was thinking, maybe 30 at best. Im just trying to be sure that my WOT rpm is in range. Thanks for the response. I've got a service tach now that Im going to compare with what's on the boat.

 

[Texasmark]

Chris1956 is better on these older engines that I am but my guess your ratio is what was frequently used at the midrange level which was 2.33:1. Your number was closer to that than 2:1 and there is some slop in the drive train. Don't know what alternator nor tach you have. On later model engines they used a 12 pole alternator and the setting was 6P as I recall....haven't looked at mine for years. If your idle rpms were around 700 you probably have it set correctly.

Mark

 

[Jamescenterconsole]

Thanks for the input Ill let y'all know what I find out. This is the stator I put on it has 12 poles if you count the two for the rectifier. Assuming Im counting correctly.
Mercury Marine, Mariner 174-5454K1 Stator - CDI Electronics

 

[Jamescenterconsole]

Anybody know anything about the Andy service tachs? Ive got it set to two stroke but the direction dont give much more that that.It looks like the tach on my boat is correct it is showing 820 RPMs at idle the service tach is showing about 1640 RPMs at idle the motor sounds right and I think I could hear that much difference. After ten days of pulling I finally got the prop off and the number is corroded off!!!!! Go figure. Could it possibly be the rectifier that's the only thing that I haven't replaced. Any more thoughts would be great.

 

[Chris1956]

An idle speed of 820RPM would be expected if the motor is timed correctly. 1640RPM would be real high and shifting would likely damage the gears. I would therefore think your tach is accurate, at least at idle.

 

[Jamescenterconsole]

Thanks Chris1956, that what I was thinking. Is there a way to check the timeing without having to retime the whole thing I read over it in the manual and it sounds confusing. That was part of my original question if I need to retime it or not?

 

[Chris1956]

Pretty easy to check the timing. Harder to retime it. That motor should have 4 coils and a switchbox. Remove spark plugs 2-4, install them into their wires and ground their bases. Put timing light on top plug wire, and remove prop. Advance throttle until carbs are just about to open. Use a jumper wire to spin motor, ign on, in gear, Timing should read 4-6* ATDC. Now advance throttle fully. Crank motor. Timing should be 21* BTDC.

 

[Jamescenterconsole]

Ok that sounds easy enough. Couple of questions just so Im clear.
Im assuming that the motor is not running just turning over with the jumper wire? Jump at the starter solinoid?
If im not mistaken there is a safty wire that need to be disconnected so the motor will turn over in gear? Black and yellow somewhere?
Sorry this one may be really stupid. How do I know its At Top Dead Center and Befor Top Dead Center, I thought you measured that in the #1 cylender with the plug removed
thanks again Chris1956

 

[Chris1956]

You will be cranking the motor in gear, which is why I recommend you remove the prop. The motor may fire a few times, but shouldn't start. When you jumper the solenoid, you bypass the neutral interlock switch.

The timing marks should specify BTDC TDC and ATDC values. Set the top piston to top dead center with a screwdriver. See if the timing pointer indicates TDC.

 

[Jamescenterconsole]

Ok so I think I have my RPM issue fixed. I replaced all four Ing. coils and took it out and it ran fine 5200 on the boat tac and 5000 on the service tac. Speed was better also. All this however has raised another question.
When I got back I pulled the plugs and found that number four still does not look like it has burned as much as the the other three. 1-3 all have a dark brown to black spot and 4 is very light. Is it possible that 4 is not getting the fuel it should? If im looking at things correctly 3 and 4 both run off the same carb. I know it has spark becaused I checked it it after I replaced the coils. 4 is on the right.